Comparative evaluation of disc diffusion and broth microdilution methods for aztreonam/avibactam susceptibility testing in Enterobacterales.

Background: Aztreonam/avibactam is a novel β-lactam/β-lactamase inhibitor (BL/BLI) combination active against carbapenem-resistant Enterobacterales (CRE), including MBL-producing isolates. In May 2024, EUCAST published Enterobacterales breakpoints for aztreonam/avibactam. This study aimed to assess the performance of commercially available disc diffusion (DD) against broth microdilution (BMD) using the latest EUCAST breakpoints.

Methods: We tested 278 CRE causing infections in 17 Greek ICUs between 2021 and 2023, using 30/20 μg aztreonam/avibactam discs and BMD according to EUCAST methodology and EUCAST version 15.0 breakpoints.

Results: Most isolates were identified as Klebsiella pneumoniae (97.8%), and 98.9% produced carbapenemases, including 46.4% KPC, 20.1% NDM, 5.4% VIM and 27% multiple carbapenemases. Using BMD, 94.2% of isolates were susceptible to aztreonam/avibactam. Conversely, using DD, 66.9% were susceptible, 33.1% resistant and 27% within the area of technical uncertainty (ATU). Most isolates in the ATU were KPC-producing (68%) or KPC and MBL-producing (29.3%). All isolates in the ATU (22-24 mm) tested susceptible by BMD (MIC ≤ 4 mg/L). One isolate exhibited resistance by DD (20 mm), but was susceptible by BMD. Categorical agreement (CA) was 72.7%, with 29% major errors (MEs) and 0% very major errors (VMEs). Using a breakpoint of 22 mm, CA, ME and VME were 99.6%, 0.4% and 0%, respectively.

Conclusions: Aztreonam/avibactam showed potent in vitro activity against MBL- and KPC-producing Enterobacterales. Using the 2025 EUCAST breakpoint, all isolates in the ATU tested susceptible by BMD, leading to high MEs of the DD method. A 22 mm breakpoint would have corrected this discrepancy in our cohort.